Fix locale data path after removing AM_GLIB_GNU_GETTEXT

Commit c7a1cf3c removes the usage of AM_GLIB_GNU_GETTEXT, but a variable set by it, DATADIRNAME, is used in libgnome-desktop/Makefile.am to specify the path to locale data, which causes bindtextdomain to be called with a wrong path so all translations are ignored. To fix it, we simply set GNOMELOCALEDIR to localedir.

I just found this problem when GNOME Shell showed date in the wrong format today. I don't know why gnome-desktop used ${prefix}/${DATADIRNAME}/locale instead of localedir, but I guess the reason was that localedir was not available: https://bugzilla.gnome.org/show_bug.cgi?id=83623.